The use of multiple antennas at both the transmitter and receiver of a wireless communication system along with related baseband signal processing is referred to as Multiple-Input Multiple-Output (MIMO) technology. MIMO can provide two forms of gains: multiplexing gain and diversity gain. Spatial multiplexing gain is more suited for the high Signal-to-Noise Ratio (SNR) scenario. In this case, multiple independent data streams are separated only in the spatial domain, but otherwise occupying the same temporal and frequency resources. The receiver employs multiple receive antennas and baseband signal processing to separate these data streams, which are combined by the radio channel. Diversity gain, on the other hand, is realized by transmitting identical information separated in the spatial domain (e.g. on different transmit antenna) which improves the robustness of the transmission. Diversity gains are beneficial when the receiver experiences low Signal-to-Noise Ratio (SNR) due to severe propagation loss, multi-user interference etc.
MIMO technology can be applied to transmit from a single transmitting station to a single receiving station. This scenario is known as Single-User (SU) MIMO. Alternatively, MIMO technology may be applied to transmit from one transmitting station to multiple receiving stations. This application is known as Multi-User (MU) MIMO or MIMO Broadcast. In a MU-MIMO system, precoding is applied at the transmitter in order to suppress mutual interference experienced by each receiving station caused by transmissions to other receiving stations. To those skilled in the art, MU-MIMO precoding, refers to spatial encoding of the transmitted signal based on propagation channel.
In order to apply MU-MIMO precoding, the transmitting station is required to know the Channel State Information (CSI) of the radio channels connecting it to each of the receiving stations for transmission. It is possible, in theory, for the transmitting station to directly measure required CSIs in Time Division Duplex (TDD) systems. Due to practical limitations, however, such as the lack of calibration between transmitting and receiving RF chains, the transmitting station is generally unable to directly measure the CSIs. Consequently, it is common to require the receiving stations to measure the CSIs and feed this information back to the transmitting station via a feedback channel. The content of the CSI feedback information is related to the amount of multipath dispersion of the channel relative to the transmission bandwidth. In addition, the CSI feedback information varies continually due to relative motion between the transmitting and receiving stations and other environmental changes such as human movement. Therefore, the CSI feedback information needs to be updated regularly. In a typical wireless system consisting of multiple stations, this CSI feedback requirement amounts to very significant signaling overhead.